A campaign to vaccinate people at risk of developing Ebola in the latest outbreak in the Democratic Republic of the Congo could begin by the end of this week, Tedros Adhanom Ghebreyesus, the director-general of the World Health Organization, said Sunday.

Tedros said the government of the DRC has formally asked to use an experimental vaccine being developed by Merck. The WHO has a stockpile of 4,300 doses of the vaccine in Geneva; the company also has 300,000 doses of the vaccine stockpiled in the United States. Merck has given its permission for the vaccine to be used in this outbreak.

“Everything is ready for the vaccine. They want it,” Tedros, who goes by his first name, told STAT in an interview from Kinshasa.

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The WHO and its partners are responding quickly, concerned that this outbreak has the potential to spread because of its location. The epicenter, a town called Bikoro, is difficult to reach by vehicle because of poor roads between it and the regional capital, Mbandaka. Tedros and his party traveled there by helicopter.

But the town is a port, on Lake Tumba. And it feeds into the Congo and Ubangi rivers — major waterways that connect to several large centers.

To the south is the DRC capital, Kinshasa (population 11.5 million), as well as Brazzaville (population 1.9 million), the capital of the neighboring Republic of the Congo. To the north is Bangui (population 800,000), the capital of the Central African Republic. Mbandaka, with a population of about 1 million people, is also reachable from Bikoro by water.

The World Food Program has established an air bridge, a costly undertaking but one that is essential for moving people and materiel into Bikoro.

The equipment needed to keep the vaccine at subzero temperatures — the so-called cold chain — was arriving in the DRC on Sunday and would be set up in the next couple of days, he said. By Wednesday or Thursday, the vaccines in Geneva will be sent to the DRC, said Tedros. After that, vaccination of health care workers and people who have been in contact with cases will begin.

“That’s our plan. And so far things are going as planned,” he said, expressing hope that the quick response will speed containment of the outbreak. “We have better weapons this time.”

The outbreak, which is believed to have started at least five weeks ago, was officially declared on May 8 after the DRC health ministry confirmed two positive tests from among a number of suspected Ebola cases in Bikoro and a village called Ikoko-Impenge, about 40 miles away.

The WHO said on Saturday the case estimate is up to 39 — two confirmed, 20 probable cases, and 17 suspected. At least 18 of those people have died. Three health care workers are among the cases and one has died.

The plan is to employ a ring vaccination approach, vaccinating anyone who has been in contact with a case to prevent continuing spread of the virus. Contact tracing efforts are already underway. To date, 382 contacts have been identified, Tedros said.

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The WHO director-general and Dr. Peter Salama, his deputy director-general for emergency preparedness and response, flew to the DRC on Friday for meetings with government officials. On Sunday, they traveled with the country’s health minister, Dr. Oly Ilunga, to Bikoro to view operations and meet with response teams and local leaders.

Tedros said multiple response partners are already on the ground at Bikoro, including Doctors Without Borders, which takes the lead on treating people with Ebola, and the International Federation of the Red Cross, which takes the lead on safe burial practices.

Tedros said the DRC government has provided all the needed documentation to trigger use of the Merck vaccine, including an import permit.

As the vaccine — provisionally called V920 — is not yet licensed, the government had to agree to deploy it under a compassionate use protocol. At this stage, it can also only be used in the context of a clinical trial — plans for which are already in the works.

The director-general noted that DRC has lots of experiencing combating Ebola. The first known outbreak, in 1976, happened there. This outbreak marks the ninth known time Ebola has broken out in the DRC.

The WHO and others have deployed many experts to the area, including epidemiologists, people skilled at contact tracing, and anthropologists who are already learning about local customs. That work is vital both to encourage people to agree to be vaccinated and to figure out if any local practices are increasing the risk the virus will spread.

This article has been updated with information about the Ebola outbreak and the vaccine.